These techniques compress the video stream to be coded in accordance with a predictive and hierarchic layered scheme: the video stream is divided into a base layer and one or more refinement layers, each nested within a layer of higher level. Each layer combined with information contained in the layer of the higher level makes it possible to improve the image frequency of the decoded stream, its spatial resolution and/or its quality. One such technique is the Scalable Video Coding (SVC) technique currently in the process of standardization by the Joint Video Team (JVT), an alliance between the MPEG (Moving Picture Expert Group) standardization group (IS O/IEC JTC1/SC29/WG11) and the International Telecommunications Union (ITU-T SG16 Q.6), in the form of an extension of the International Standards Organization (ISO) standard 14496-10, also known as the H.264/MPEG-4 AVC (Advanced Video Coding) standard.
FIG. 1 shows an example of a data stream compressed in accordance with this standard. It consists of a base layer CB and two refinement layers CR1 and CR2 associated with the base layer CB. The refinement layers CR1 and CR2 code data making it possible to enhance the quality, spatial resolution or frequency of the coded images in the base layer CB, which is a stream coded to the ABC standard, for example. The SVC standard also provides for segmenting these layers CB, CR1, and CR2 into temporal levels NT1, NT2, and NT3.
Such techniques make it possible for terminals with different capabilities, such as fixed personal computers, mobile telephones, and personal digital assistants (PDA), to receive the same content broadcast over a communications network such as the Internet and to decode it as a function of their respective capabilities. Thus a low-resolution mobile terminal decodes only the base layer of content received via the transmission network, whereas fixed terminals of greater capability decode the base and refinement layers of the content, thereby displaying the content with maximum quality. These scalable coding techniques therefore make it possible to save on bandwidth, a single content being broadcast to terminals with different capabilities instead of one content being broadcast for each type of terminal.
Moreover, some contents also require adaptation as a function of the profile of the user receiving them. For example, a content provider seeking to promote a video program often broadcasts the entire program to subscribers to the program and broadcasts only a portion of the program to non-subscribers or broadcasts it to them in encrypted form. Similarly, some programs are adapted to suit persons with a hearing impairment: an interpretation in sign language is embedded in the broadcast images for them. Other programs further require regional differentiation; for example, the news carries different stories as a function of the regions in which it is broadcast.
Scalable image coding techniques as described above provide no specific processing for personalizing the same video stream. Accordingly, to personalize a video stream as a function of different profiles, a content server broadcasting the video stream in a communications network must at the head end code and broadcast as many scalable video streams, for example as many SVC streams, as there are user profiles. This therefore requires as many multimedia session negotiations between the content server and one of the points of presence redistributing this stream to the users in the communications network as there are user profiles, and likewise as many assignments of resources between these two network entities as there are user profiles. This implementation is not the optimum, because the multimedia sessions negotiated in this way and that have to be maintained are costly to create and to maintain even though they have substantially identical characteristics:                start and end of multimedia session;        necessary bit rate;        coding of base and refinement layers;        distribution core network, etc.        